site/wiki/pages/gemini-cgi.md

author	published	title	description	category
~khuxkm	true	gemini cgi guide	a guide to using CGI on gemini	main

where to put CGI scripts

you can put CGI scripts anywhere. as long as you make the file executable and have it output the correct format for gemini. the server calls a new instance of the program for every request it has to handle, meaning that data isn't saved between runs unless you manually save the data to a file or database and load the data on the next run.

what languages can be used to write CGI

any language that can read environment variables can be used for CGI. even shell scripting can be used for CGI, given you provide the correct shebang.

how to write a gemini CGI script

these are the environment variables gemserv (our trusty gemini server) gives a CGI program:

• GATEWAY_INTERFACE - always CGI/1.1 in our case
• GEMINI_URL - the full URL used to request the page, including any query strings
• PATH_INFO - any extra path after the script itself (for instance, gemini://tilde.team/~khuxkm/test.cgi/test/one gives a PATH_INFO of /test/one)
• REMOTE_ADDR, REMOTE_HOST, REMOTE_PORT - the addr/host/port of the connecting client, the former two useful for rate-limiting
• SCRIPT_NAME - the path component of the URL that corresponds to the script being called
• SERVER_NAME - the name of the server, in our case tilde.team
• SERVER_PROTOCOL - always GEMINI in our case
• SERVER_SOFTWARE - always gemserv in our case
• LC_CTYPE - always C.UTF-8 in our case (allows for UTF-8 output)

also, if the user supplies a cert:

• AUTH_TYPE - always Certificate if present; if not present, no cert is supplied
• TLS_CLIENT_HASH - the hash of the certificate, prepended with the hash type
• REMOTE_USER - the Common Name of the certificate

in order to write a CGI program, simply use these environment variables to figure out what you want to do and then do it. for a simple hello world:

#!/bin/sh
printf '20 text/gemini\r\n'
printf 'Hello world from CGI!\r\n'

do note that the first line MUST use \r\n as a terminator. after that, if you're serving gemtext, you can use whatever LF/CRLF you want for line endings as long as you stay consistent.

for a more complex hello world that asks for input, here's a python script that asks for your name and says hello to you:

#!/usr/bin/python3
from urllib.parse import unquote
from os import environ
from sys import exit

if "QUERY_STRING" not in environ:
    print("10 Please enter your name",end="\r\n")
    exit()
name = unquote(environ["QUERY_STRING"])

print("20 text/gemini",end="\r\n")
print(f"Hello {name}!")

remember, anything that can read environment variables and write to stdout can be used to make CGI, as long as the server can figure out how to execute it.